The present invention relates to a fuel cell using a molten carbonate electrolyte and a method of manufacturing an electrolyte plate for use in the fuel cell.
A fuel cell which utilizes molten carbonate as electrolyte and operates at a high temperature (500.degree. C.-800.degree. C.) has been used widely because there is no need of use for expensive catalysts such as plutonium which is necessary in a fuel cell operating at normal temperature and it is possible to obtain a high current density even if inexpensive fuel, whose reaction is usually hard to occur at normal temperature, is used.
An example of the structure of such molten carbonate fuel cell is shown in FIG. 1, in cross section. In FIG. 1, the fuel cell comprises an electrolyte plate 1, an anode electrode 2 of porous nickel or nickel alloy disposed on one surface of the electrolyte plate 1, a cathode electrode 3 of porous nickel oxide disposed on the other surface of the electrolyte plate 1 and a pair of cell frames 4 for defining a fuel gas chamber 10 and an oxidation agent chamber 11 in the sides of the anode electrode 2 and the cathode electrode 3, respectively. These components are assembled between a pair of pressing plates 5 through insulating plates 6 and fixed together by bolts 7 and nuts 8 as shown. The fixedly assembled fuel cell is put in a furnace to heat it up to a temperature around 650.degree. C. and fuel gas and oxidizing agent are supplied continuously to the fuel gas chamber 10 and the oxidizing agent chamber 11, respectively. Electric energy produced between the electrodes 2 and 3 proportionally to areas thereof is derived through lead wires 12 connected suitably to the electrodes.
When alkali metal carbonate is used as the electrolyte and the fuel cell is operated at a temperature in a range from 500.degree. C. to 800.degree. C., an electrochemical reaction therein proceeds at a high reaction rate according to the following formula and the ion conduction is performed by carbonic ions (CO.sub.3.sup.2-).
anode side: H.sub.2 +CO.sub.3.sup.2- .fwdarw.H.sub.2 O+CO.sub.2 +2e PA1 cathode side: 1/2O.sub.2 +CO.sub.2 +2e.fwdarw.CO.sub.3.sup.2-
However, the operating temperature is so high and alkali metal carbonate is so corrosive that many problems that occur in the electrodes, the electrolyte plate and other elements such as the cell frames etc. For example, there may be degradation of electrochemical chracteristics of the electrode due to growth of nickel particles constituting the electrodes and corrosion thereof, breakage of the electrolyte plate during assembling thereof or due to heat cycle thereof, degradation of electrolyte holding capability thereof, and corrosion of other constituent elements. Inter alia, these problems occuring in the electrolyte plate affect the performance of the fuel cell substantially. That is, defects of the electrolyte plate, such as holes or cracks may cause undesirable mixing of fuel gas and air during the operation of the cell, which causes an output performance of the fuel cell to be lost.
In order to improve the performance of the fuel cell, the electrolyte plate of such molten carbonate fuel cell should have a large area and should satisfy all of the requirements of high mechanical strength, high heat cycle durability, high heat-resistivity, high stability of holding electrolyte and high ion conductivity.
On the other hand, the cost of the electrolyte plate occupies a high percentage of the total cost of the fuel cell. Therefore, in order to reduce the total cost, the cost of the electrolyte plate should be restricted to as low as possible.
The electrolyte plate comprises an electrolyte holding member and electrolyte such as alkali metal carbonate to be held thereby. As the electrolyte holding member, lithium aluminate produced by mixing alumina and lithium carbonate according to a carbonate mixing method etc. has been considered as an optimum material therefor. Lithium aluminate includes three isotopes, i.e., .alpha.-lithium aluminate, .beta.-lithium aluminate and .gamma.-lithium aluminate. Particularly, .beta.-lithium aluminate or .gamma.-lithium aluminate is used for the electrolyte holding member in view of the mechanical strength thereof.
As the electrolyte, a mixture of lithium carbonate and potassium carbonate which are eutectic materials has been used among other alkali metal carbonates. The eutectic constitution ratio of them in weight percentage is 47.5% lithium carbonate -52.5% potassium carbonate and the eutectic temperature is about 491.degree. C. Further, the weight ratio of the electrolyte to the lithium aluminate electrolyte holding member is usually 50% to 60%.
Such an electrolyte plate may be manufactured according to various methods including the so-called paste method in which a mixture of .gamma.-lithium aluminate powder and electrolyte powder containing eutectic materials is press shaped at normal temperature and then sintered at about 500.degree. C., the so-called hot press method in which the mixture powder is pressed under 0.6-1.0 ton/cm.sup.2 at 460.degree. C.-490.degree. C. for 15-150 minutes and the so-called matrix method in which .gamma.-lithium aluminate powder, together with a binder, is press shaped under 1-3.5 ton/cm.sup.2 and then sintered to form a matrix and the latter is immersed in a molten electrolyte.
An electrolyte plate manufactured according to the paste method is easily cracked by increased pressure. Further, in order to avoid cracking during the sintering, increasing and decreasing rates of temperature must be selected very carefully, causing the producibility thereof to be low. In addition thereto, since the bulk density of the electrolyte plate obtained by this method is about 85% of the critical value at most, the mechanical strength thereof is low and thus it is easily cracked or damaged during the heat cycle of the full cell.
The hot press method is advantageous over the paste method in that it is possible to increase the bulk density of the electrolyte plate and the mechanical strength thereof is higher. However, the improvement of the bulk density requires an increased pressing force and therefore a large press machine is necessary, which is very expensive. It is also necessary in the hot press method to select the varying rates of temperature very carefully. In addition, these methods are not suitable to provide an electrolyte plate having a large area.
The matrix method includes the so-called doctor blade method, the calender method and the electrophoresis method etc. Although these methods make the area of the electrolyte plate larger easily comparing with the paste method or the hot press method, the manufacturing process of each of them is complicated and includes a high temperature sintering step, causing the manufacturing cost to be high and the matrix in the pressing step exhibits a very low mechanical strength and the resultant matrix is fragile. In addition, since the binder contains toxic organic substances, special considerations must be taken for workers' safety.
Therefore, none of the electrolyte plates manufactured by these methods satisfy all the requirements mentioned previusly and thus the molten carbonate fuel cell equipped with the electrolyte plate manufactured by any of the methods exhibits an insufficient output power. Since the mechanical strength, heat resistivity and heat cycle durability etc. of the connectional electrolyte plate are low, the fuel cell must be heated to its operating temperature of 650.degree. C. at a rate as low as 60.degree. C./hour, otherwise the electrolyte plate may be damaged.
As another method of manufacturing such a ceramic porous sheet, a method is known in which the foaming of soft urethane is utilized to form continuous pores in the ceramic sheet. The ceramic sheet obtained according to this method is usually insufficient in view of hardness and density and, when filled with electrolyte, provides insufficient ion mobility and electron conduction. Even if the problems of ion mobility and electron condition are resolved, electrolyte therein easily leaks away because the average size of the pores is considerably large.
According to a further conventional method, a mixture of .alpha.-alumina and 50-70 wt% wood pulp is shaped into a sheet and the sheet is burned. According to this method, a thin and dense porous sheet can be obtained. However, due to the uses of .alpha.-alumina and such large amount of wood pulp selected in order to obtain a high porosity, the burning must be performed at a temperature as high as 1500.degree. C. to 1600.degree. C., otherwise sufficient mechanical strength of the sheet can not be obtained.
This method corresponds to that described in Japanese Unexamined Patent Application Publication No. 33273/1985 published Feb. 20, 1985. In this reference, a porous plate prepared by sintering a green sheet containing a large amount of wood pulp (51 to 70 wt%) has pores, the size of which are too large to hold molten carbonate electrolyte, making it impossible to use in a molten carbonate fuel cell (MCFC).
U.S. Pat. No. 4,526,845 and U.K. Patent Application No. GB 2 108 099 A use lithium aluminate in fabricating a porous plate. However, the porous plate of these two references is prepared by burning at about 600.degree. C. At this temperature, lithium aluminate cannot be sintered, the melting point of lithium aluminate being 1625.degree. C. Thus, these plates have no self-supporting nature. Further, U.S. Pat. No. 4,526,845 uses a temporary plastic binder such as polyvinyl butyral in the form of particles. Thus, any pores obtained by burning the binder are spherical in shape even though they are communicated with each other. Also, because the .gamma.-lithium aluminate is not sintered, the resultant spherical pores correspond in shape to the binder particles.
Thus, in U.S. Pat. No. 4,526,845, the .gamma.-lithium aluminate which is in the form of inert particles and which is 40-45% of the volume of the matrix before burning, is not sintered. Further, the matrix tape is heated up to 650.degree. C., which is as low as the operating temperature of the fuel cell. Therefore, the matrix of U.S. Pat. No. 4,526,845 might take the form of an electrolyte plate obtainable by the hot press method. However, the rigidity is then maintained by filling of the gaps between particles with electrolyte. The rigidity is not maintained by the mutual adhesion of lithium aluminate particles. Thus, the plate is not durable against a heat cycle.
Because the mechanical strength of this matrix is low and so easily broken, the bubble barrier layer is laminated to prevent gas-crossover through cracks of the matrix due to the heat cycle. That is, the mechanical strength of the matrix of U.S. Pat. No. 4,526,845 depends upon the nature of the barrier layer.
An injection molding, an extrusion molding and a hydrostatic pressing are other methods of manufacturing the ceramic sheet.
In any of the conventional methods it is very difficult to manufacture a satisfactory ceramic sheet having a area of 30 cm.sup.2 or more.